Lecture 13 (Topology)

In this lecture, how is the boundary ∂A of a subset A of a topological space X defined?

1. A.∂A = cl(A) − int(A)
2. B.∂A = int(A) − cl(A)
3. C.∂A = cl(A) ∪ int(A)
4. D.∂A = X − cl(A)